The paper industry has been practicing waste paper recycling to regenerate usable cellulosic fiber for paper making for many decades. In these processes, ink is removed from the waste paper pulp using a suitable deinking composition. By controlling the deinking process, a recycling mill can affect the properties of the paper such as the brightness and can improve the usability of the cellulosic fiber for paper manufacturing.
Deinking consists of a series of complex chemical and physical processes. These events include but are not limited to ink detachment, ink dispersion, ink collection, ink transport, and removal of inks from the waste paper pulp slurry. Each of these microprocesses have different surface and interfacial demands within the recycling operation in order to efficiently and effectively deink waste paper and produce quality paper.
Conventionally, two different methods have been employed to isolate the ink and ink related entities in order to produce the deinked fiber after repulping. These two processes are flotation and wash deinking. Often, processes contain both flotation and wash deinking and can be referred to as combination deinking processes. The underlying chemical and physical requirements to successfully deink are different for wash, flotation, and combination deinking processes.
More specifically, flotation/washing combination deinking refers to a deinking process wherein the ink released by the deinking composition is separated from the cellulosic fibers primarily through the flotation devices or flotation cells of the recycling process prior to passing the pulp through washing stages. Alternatively, washing/flotation combination deinking refers to a deinking process wherein the ink released by the deinking composition is separated from the cellulosic fibers primarily through the washing devices of the recycling process prior to passing the pulp through flotation stages. Wash deinking, on the other hand, refers to a deinking process wherein the ink released by the deinking composition is separated from the cellulosic fibers in the washing stages.
Flotation processes are fundamentally different than washing processes. This difference is partly because the ink size and hydrophobicity are important for favorable separation. Flotation methods of ink removal generally involve passing air bubbles through an aqueous system containing dispersed cellulose fiber that is most often produced via a repulping process. The resulting repulped cellulose fiber slurry having therein additives added either before, during, or after repulping. As the air bubbles rise within the fiber slurry and carry the ink particles with them, they generate foam which is enriched in ink that is subsequently removed from the flotation cell. The amount of foam that is typically favored in flotation deinking systems is an amount which can be collected by skimming, decanting, or other means, and which transports with it an enriched concentration of ink while minimizing the amount of other solids such as fibers that are rejected.
Flotation deinking usually utilizes different surfactants than washing because the resulting surface properties and size of the ink particles that is beneficial for flotation deinking is different than is desirable for wash deinking. Examples of traditional nonionic surfactants that may be used in the flotation deinking process include alkylene oxide adducts of fatty alcohols, alkylphenols, fatty acids, and alkanolamides. Such nonionic surfactant deinking agents may be used either by themselves or in combination with one another and can also be blended with nonalkoxylated fatty acids and fatty alcohols. Flotation deinking has historically relied on a high pH pulping liquor to remove ink from the fiber and facilitate efficient usage of pulper bleach additives.
Wash deinking typically requires fine dispersion of ink. The ink and fiber are uniformly distributed throughout the slurry, and foaming or bubble formation, though present, is not particularly desired. The objective in the washing process is to release the ink from the fiber into an aqueous medium and then separate the fiber from the aqueous medium. Thus, the washing method comprises repulping, preferably under low-foaming conditions, of secondary fiber in an ink-removing aqueous medium whereby the ink (and other non-cellulosic contaminants, if present) is mechanically and/or chemically removed as desired from the fiber. The repulping step is typically followed by dilution and/or screening. Certain surfactants are known to those skilled in the art to modify interfacial properties successfully for wash deinking. These surfactants, however, also modify the ink into a hydrophilic dispersed state that favors the washing method.
Washing and flotation processes both depend on the proper use of surfactant. Depending on the relative contributions and characteristics of the hydrophilic and hydrophobic portions of the surfactant molecule, the surfactant's interaction with the ink will vary as ink particles will be rendered either hydrophilic for washing purposes or more hydrophobic for flotation. The opposing natures of wash surfactant and flotation surfactant can lead to inefficiency in combination deinking systems. The deinking mechanism for washing is quite different from that for flotation and, therefore, they require the use of deinking compositions having different properties.
Typically, either a flotation-derived or washing-derived deinking chemistry is utilized in a given deinking system. At some point in either process, the deinked, repulped waste paper is often passed through a series of fine cleaners and/or screens where the small particulate contaminants (e.g., sand and grit) are removed. Additional processing stages may be required such as, for example, dispersion, to reduce the particle size of any contaminants, or a special cleaning stage with special cleaners designed to remove specific contaminants.
The chemistry involved in traditional deinking very often involves addition of caustic soda in the repulper to increase the pH, often greater than 9 and sometimes greater than 10. Increasing the pH, however, often causes yellowing and darkening of the waste paper stock—especially when the waste paper contains groundwood or mechanical pulps. To counteract this undesirable darkening effect, a bleaching additive is typically added to increase the whiteness and brightness of the pulp. The deinked waste paper is then held in storage until it is eventually fed to a papermaking machine.
Surfactant-based deinking aids, especially nonionic surfactants, can be excellent ink detachment agents. Some nonionic surfactants assist in ink collection. If not properly chosen, however, these additives can also actually hinder ink collection in flotation containing processes. It is well known that traditional fatty acid soaps are efficient ink collectors in alkaline systems (pH greater than 9), although these soaps can demonstrate decreased ink detachment characteristics and lead to deposit concerns later in the process.
Because of the relatively low deinking collection efficiency and high cost of various deinking aids, many paper mills have become technically and/or economically motivated to explore new deinking programs. One such solution for improved deinking performance is obtained by using a blend of fatty acid soaps and nonionic surfactants. These blends are often found to demonstrate superior performance in deinking waste paper slurries. These blends often lead to a lower consumption of fatty acid soaps and are useful for mills that are technically or economically motivated to use less or no soap.
Strategies for improved collection efficiency, however, depend on factors other than the deinking chemistry. Specifically, the collection efficiency is strongly influenced by the type of recovered paper raw material and printing method. In old newsprint (“ONP”)-containing systems the deinking chemistry is crucial to determining the collection efficiency and ultimately the ink removal and final deinking pulp quality.
There is a need for a more efficient and cost effective method to collect ink during the process of deinking waste paper.
There is also a need for a method that produces paper pulp that has both excellent brightness and low effective residual ink concentrations (“ERIC”).
There is also a need for a method that improves final paper pulp quality and paper pulp yield during flotation/washing combination and wash deinking processes.
There is also a need for improved ink collection and removal in reduced alkali and/or neutral conditions.